Computer-to-computer data networking today is a very diverse and broad field. It has traditionally been divided into two categories: Local Area Networks (LANs) and Wide Area Networks (WANs). The following characteristics have classically distinguished LANs from WANs:
LANs exist within a single customer site, with the physical medium itself (copper twisted pair cabling, coaxial cabling and/or fiber optic cabling) being owned by the customer and used exclusively for the LAN service. LANs have generally been connectionless in nature (all users attached to a LAN are implicitly assumed to be able to access each other) and usually support multipoint messages (one user simultaneously talking to many users) as well as point to point messages.
WANs interconnect customer sites across wiring infrastructures owned by telecommunications providers (examples: AT&T, MCI, Nynex, US West etc.); these wiring infrastructures are generally also used to carry voice services (telephone). WANs have generally been circuit-oriented in nature (communications take place only after an explicit connection between two communicating users is established) and usually do not support multipoint messaging easily.
Historically, WANs speeds have been very low as compared to LANs due to limitations of the telecommunications infrastructure. WAN costs have also been very high, limiting the deployment of WANs to interconnection of large customer premises sites. However, three factors have together created new demands for WAN connectivity and services:
Increased use of LANs within enterprises has made it imperative that all employees of the enterprise have access to the LAN, even those located in small remote offices.
More and more workers are "telecommuting"--working from home--and require access to the enterprise LAN.
Rapid expansion of the Internet has created consumer demand for network access to the home.
The classic WAN configuration--interconnection of primary enterprise sites--is now referred to as a "Backbone WAN". WAN configurations which support frequent, intermittent access by remote users in the categories above are termed "Access WANs". The invention relates primarily to Access WAN configurations.
The Access WAN is constrained to operate over the existing telecommunications infrastructure. There are alternatives proposed which utilize the cable television infrastructure. For most households and small offices, this means the access WAN must operate over the existing copper wiring (generally two pairs--twisted pair, termed the subscriber loop) which provide voice service today. This is a significant constraint for the technologies used.
Multiple technologies exist and are proposed for implementation of Access WANs. The most common of these is the analog modem. With an analog modem, users can dial into access server devices in enterprises and establish connections over the existing telephone network, at data rates up to about 30 kilobits/second. This has become the most common technique for telecommuters and Internet users. However, the speeds achievable with analog modems appear to be reaching fundamental physical limits, and only provide marginal service for data-intensive network transactions.
Integrated Services Digital Network, or ISDN, is an alternative technology which is frequently used in Access WANs. ISDN enables bidirectional communication at 64 or 128 kilobits/second, offering somewhat better performance than analog modems. However, telecommunications providers in the United States have been slow to deploy ISDN due to the large infrastructure upgrades required, and both availability and pricing of the service have suffered. ISDN has also been problematic for users due to its high complexity.
Asymmetric Digital Subscriber Loop, or ADSL, is a technology proposed for use in Access WANs. ADSL is a high-speed modem technology originally developed for Video-on-Demand services. It enables a full-duplex communications path on existing copper wiring with an asymmetric data rate: the data path from the subscriber to the Central Office operates between 64 and 640 kilobits/second, while the data path from the Central Office to the subscriber operates between 1.5 and 9 Megabits/second. Significantly, ADSL may operate over the existing telecommunications infrastructure without substantial investment, and is transparent to voice services.
With the use of ADSL, a high-speed bidirectional communications path may be established between a household or small office and the local telecommunications provider's Central Office. This is a critical building block for an Access WAN but does not provide a complete system; many additional decisions are now required to build an end-to-end interoperable solution. An ad-hoc standards body called the ADSL Forum has been formed to work on these standards.
Implementation of an ADSL-based Access WAN fundamentally requires three elements:
1. An interface device which resides in the customer premises and connects local data traffic to the ADSL link. This must include the ADSL modem technology and intelligence to convert local data traffic to the appropriate format.
2. An aggregation device which resides in the central office and integrates many ADSL links together for presentation to the inter-Central Office WAN backbone.
3. A common convention between the two devices above for data formats to be used on the ADSL link, as well as a consistent communications paradigm (LAN-oriented connectionless/multicast versus WAN-oriented circuit-based).
It is generally assumed that the WAN backbone between central offices will be implemented using dedicated or switched virtual circuits over a connection-oriented technology such as Asynchronous Transfer Model (ATM). Similarly, it is generally assumed that within the customer premises (such as a small office), there will be a small Ethernet LAN (connectionless and supporting multicast messaging).
The ADSL Forum has outlined three potential models for communication between these disparate networks:
A. Transfer data over the ADSL link using Ethernet data formats, and convert the data to WAN formats in the Central Office.
B. Transfer data over the ADSL link using Frame Relay data formats; conversion from Ethernet to Frame Relay is done in the customer premises.
C. Transfer data over the ADSL link using ATM data format; conversion from Ethernet to ATM is done in the customer premises.
Both transfer data over the ADSL link using Frame Relay data formats; conversion from Ethernet to Frame Relay is done in the customer premises and transfer data over the ADSL link using ATM data format; conversion from Ethernet to ATM is done in the customer premises require use of a costly and complex LAN to WAN conversion device in the customer premises.